Reactor jacket design

ABSTRACT

Reactor systems, reactor coolant systems, and associated processes for polymerizing polyolefins are described. The reactor systems generally include a reactor pipe and a coolant system, in which the coolant system includes a jacket pipe surrounding at least a portion of the reactor pipe to form an annulus therebetween, at least one spacer coupling the jacket to the reactor pipe, and a coolant which flows through the annulus to remove heat from the reactor pipe. At least one of the external surface of the reactor pipe, the internal surface of the jacket, and at least one spacer, are independently modified, for example by polishing, coating, or reshaping, to reduce the fluid resistance of the coolant flow through the annulus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.16/402,678 filed May 3, 2019, which is incorporated herein by referencein its entirety.

FIELD OF THE DISCLOSURE

This disclosure relates to the coolant system design for a reactorjacket in an olefin polymerization system and related methods.

BACKGROUND

Specific types of polyolefins, such as high-density polyethylene (HDPE),have a number of use-specific applications in the manufacture ofblow-molded and injection-molded goods, such as food and beveragecontainers, film, and plastic pipe. Other types of polyolefins, such aslow-density polyethylene (LDPE), linear low-density polyethylene(LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene(sPP) are also suitable for similar, and a variety of other,applications. The mechanical requirements of the specific application,such as tensile strength and density, and/or the chemical requirements,such as thermal stability, molecular weight, and chemical reactivity,typically determine what type of polyolefin is most suitable.

To satisfy the demand for such a wide range of polyolefins, variousprocesses have been developed by which olefin monomers such as ethylene,propylene, butene, pentene, hexene (1-hexene), octene, decene, andothers may be polymerized to form polyolefin. Olefin polymerization mayoccur in a liquid-phase polymerization reactor and/or gas-phasepolymerization reactor to form the polymer (polyolefin) solidparticulates as “fluff” or pellets. The reaction conditions such astemperature, pressure, chemical concentrations, polymer production rate,and so forth, may be selected to achieve the desired melt, physical,rheological, and/or mechanical properties such as density, melt index(MI), melt flow rate (MFR), copolymer content, comonomer content,modulus, and crystallinity, which are desired for the specificapplication of the polyolefin.

Therefore, there is an ongoing need for new methods and systems that canprovide improved performance in the polymerization process, for example,in catalyst activity, monomer yield, energy efficiency, diluentrecovery, increased throughput, and so forth. Those improvements whichmay enhance production while generating cost savings. For example,capital and energy costs are both impacted by the efficiency of thecoolant system used to remove the heat of reaction from the reactorsused in the polymerization process, and there remains a need for moreefficient and effective methods and systems for removing heat from thepolyolefin process.

SUMMARY OF THE DISCLOSURE

In an aspect, this disclosure provides reactor systems, reactor coolantsystems, and associated processes for polymerizing polyolefins. In anaspect, the reactor systems generally include a reactor pipe such as aloop reactor pipe, which is jacketed in one or more locations by anouter coolant system jacket pipe, which is spaced apart from the reactorpipe. In this configuration, the combination of the reactor pipe andjacket pipe forms an annulus between an internal surface coolant systemjacket pipe and an external surface of the loop reactor pipe. Thecoolant system includes at least one, typically a plurality of spacerswhich couple and secure the jacket pipe to the reactor pipe. Coolantfluid flows through the annulus to remove heat from the reactor pipe.

In this coolant system arrangement, the coolant fluid contacts and flowsacross not only the internal surface of the jacket pipe and the externalsurface of the reactor pipe, but also across and around the spacerswithin the annulus of the cooling system. This contact of the coolantwith these surfaces produces a resistance to fluid flow, decreasescooling system efficiency, and can increase costs associated withcooling and overall process costs.

Therefore, aspects of the present disclosure include one of more methodsand processes for improving the efficiency of such as cooling system,including but not limited to: [1] reducing the resistance to coolantfluid flow by reducing the roughness of the internal surface of thejacket pipe, the external surface of the reactor pipe, the spacerswithin the annulus, or a combination thereof, for example, by polishingor applying a friction-reducing coating; [2] shaping the spacers,particularly shaping the cross-section of the spacers, for improvedhydrodynamic performance and coolant fluid flow; [3] providingprotrusions or protuberances extending from the external surface of thereactor pipe and/or the internal surface of the jacket pipe into aportion of the annular space, including where the protrusions arematched with a corresponding protrusion on the opposite side of theannulus, to create a venture effect from the coolant flow; or [4] anycombination thereof. In an aspect, the protrusions and the spacers maybe shaped in order to impart a curved or partially curved leading edge,and a tapered or partially tapered trailing edge, with respect to fluidflow. The features and methods of this disclosure are applicable tovertical loop reactors and to horizontal loop reactors.

Therefore, in accordance with an aspect, this disclosure provides areactor system comprising a reactor pipe having an external surfacecharacterized by an unmodified surface roughness; and a coolant system,wherein the coolant system comprises a jacket having an internal surfacecharacterized by an unmodified surface roughness, the jacket spacedapart from and surrounding at least a portion of the reactor pipe toform an annulus between the internal surface of the jacket and theexternal surface of the reactor pipe, a coolant which flows through theannulus to remove heat from the reactor pipe, and at least one spacerhaving a leading edge and a trailing edge with respect to the coolantflow and characterized by an unmodified surface roughness, the spacercoupling the jacket to the reactor pipe; wherein the external surface ofthe reactor pipe, the internal surface of the jacket, the at least onespacer, or any combination thereof are independently modified to reducethe fluid resistance of the coolant flow through the annulus.

Therefore, in accordance with this aspect, by reducing the fluidresistance of the coolant flow through the annulus, the reactor systemand process disclosed herein also can provide improvements in pressuredrop across the system, leading to less energy being required to pumpthe coolant, and improvements in cooling efficiency.

In an aspect, the at least one spacer couples the internal surface ofthe jacket to the external surface of the loop reactor and generally cancomprise a material selected from ceramic, stainless steel, or acombination thereof. In another aspect, the spacer or a portion thereofcan be treated by polishing or by coating the spacer to provide a moresmooth, less rough surface which aids fluid flow.

In a further aspect of the present disclosure, there is provided areactor system comprising a loop reactor, the loop reactor comprising areactor pipe having an external surface and a volume of greater thanabout 35,000 gallons and further comprising a series of reactor pipesections which form a series of legs which form a loop; and, a coolantsystem comprising a jacket having an internal surface, spaced apart fromand surrounding at least a portion of the reactor pipe to form anannulus between the internal surface of the jacket and the externalsurface of the reactor pipe, a coolant which flows through the annulusto remove heat from the reactor pipe, and a plurality of spacers, eachhaving a leading edge and a trailing edge with respect to the coolantflow and characterized by an unmodified surface roughness, each spacercoupling the jacket to the reactor pipe; wherein the plurality ofspacers are modified by shaping each spacer in cross section to providea curved or partially curved leading edge, and a tapered or partiallytapered trailing edge, and the total coolant pressure drop through thecoolant system is less than 15 psi.

There is also provided in this disclosure a method of polymerizingolefins, the method comprising contacting at least one olefin monomerwith catalyst within the reactor system comprising the disclosed coolantsystem under polymerization conditions sufficient to form a polyolefin.The methods and features of this disclosure are applicable to, but notlimited to, loop reactors, including horizontal loop reactors andvertical loop reactors. In an aspect, a loop reactor according to thisdisclosure can operate in liquid or supercritical phases, that is, thereactor pressure can be greater than or less than the critical pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these figures in combination with the detailed description ofspecific embodiments presented herein. The drawings in this disclosureare not necessarily drawn to scale.

FIG. 1 illustrates a schematic of one embodiment of a polymerizationreactor system in accordance with the present disclosure, namely a loopreactor system comprising a series of reactor pipe sections which form aseries of legs and which form a loop, which can be used with the coolingsystem disclosed herein.

FIG. 2 illustrates a cross-sectional view of a jacketed section of aloop reactor system according to one embodiment taken along line 2-2 ofFIG. 1 , viewed perpendicular to the direction of fluid flow.

FIG. 3 illustrates a top view of a portion of a jacketed section of aloop reactor system according to one embodiment of the presentdisclosure, taken along line 3-3 of FIG. 1 , and seen parallel to thedirection of fluid flow, which is also shown in FIG. 2 for ease ofunderstanding.

FIG. 4 illustrates a cross-sectional area of an exemplary spaceraccording to one embodiment of the present disclosure taken along line4-4 of FIG. 3 , which is also shown in FIG. 2 for ease of understanding.This view is along or parallel to the length of the spacer between theloop reactor pipe 260 and the coolant system jacket 240, and viewedperpendicular to the direction of flow, and showing a curved leadingedge of the spacer.

FIG. 5 illustrates a cross-sectional area of another exemplary spaceraccording to another embodiment of the present disclosure, takenperpendicular to the length of the spacer between the loop reactor pipe260 and the coolant system jacket 240, and viewed perpendicular to thedirection of flow and showing a curved leading edge and a tapered orpartially tapered trailing edge of the spacer.

FIG. 6 illustrates a front perspective view of a section of a spaceraccording to one embodiment of the present disclosure, showing theleading edge 216 which is contacted by the cooling fluid first and thetrailing edge 211. Fluid flow in FIG. 6 is from a rounded front leadingedge toward a rounded but narrower rear trailing edge of the spacer. Thenarrower rear trailing edge of the spacer in FIG. 6 is not nearly astapered or narrow as the rear trailing edge of the spacer in FIG. 7 .

FIG. 7 illustrates a rear perspective view of a section of a spaceraccording to one embodiment of the present disclosure, showing theleading edge 216 which is contacted by the cooling fluid first and thetrailing edge 211. Fluid flow in FIG. 7 is from a rounded front leadingedge toward a much more substantially tapered and narrow rear trailingedge of the spacer as compared to the trailing edge of the spacer shownin FIG. 6 .

FIG. 8 illustrates two possible exemplary fluid flow patterns arounddifferently shaped spacers, in accordance with the present disclosure.Specifically, FIG. 8 illustrates the cross-sectional areas of twoexemplary spacers according to other embodiments of the presentdisclosure, taken perpendicular to the length of the spacer between theloop reactor pipe and the coolant system jacket pipe, and viewedperpendicular to the direction of flow.

FIG. 9 illustrates a partial, cutaway perspective view of an embodimentof the reactor system of the present disclosure, showing the reactorpipe, the jacket pipe, the annulus therebetween, and a spacer situatedbetween the inside wall of the jacket pipe and the outside wall of areactor pipe.

FIG. 10 illustrates a partial, cutaway perspective view of anotherembodiment of the reactor system of this disclosure, showing the reactorpipe, the jacket pipe, the annulus therebetween, and a spacer situatedbetween the inside wall of the jacket pipe and the outside wall of areactor pipe, in which the spacer is oriented 90° from the spacerillustrated in FIG. 9 .

FIG. 11 illustrates a cross-sectional area of a protrusion orprotuberance resembling a shaped bulge, according to an embodiment ofthe present disclosure, the bottom of which is attached to the curvedsidewall of the internal surface of the jacket or the external surfaceof the loop reactor, and which extends into the annular space. In thisembodiment, the protrusion is shaped to provide a curved leading edgeand a partially tapered trailing edge. Fluid flow is from left-to-rightin FIG. 11 , that is, from the leading edge to the trailing edge.

FIG. 12 illustrates a cross-sectional area of one portion of the annularspace between the coolant system jacket 240 and the loop reactor pipe260 and viewed perpendicular to the direction of coolant flow, accordingto an embodiment of the present disclosure This figure illustrates twoopposing protrusions or protuberances, 1200, the bottoms of which areattached to the curved sidewall of the internal surface of the jacket241 and the external surface of the reactor pipe 261, respectively, andboth of which extend into the annular space. Fluid flow is fromleft-to-right in FIG. 12 , that is, from the leading edge to thetrailing edge of the protrusions, and an exemplary venturi effect thatmay be generated between two the protrusions are shown.

While the inventions disclosed herein are susceptible to variousmodifications and alternative forms, only a few specific embodimentshave been shown by way of example in the drawings and are described indetail below. The figures and detailed descriptions of these specificembodiments are not intended to limit the breadth or scope of theinventive concepts or the appended claims in any manner. Rather, thefigures and detailed written descriptions are provided to illustrate theinventive concepts to a person of ordinary skill in the art and toenable such person to make and use the inventive concepts.

DETAILED DESCRIPTION

In the drawings and description that follow, like parts are typicallymarked throughout the specification and drawings with the same referencenumerals, respectively. In addition, similar reference numerals mayrefer to similar components in different embodiments disclosed herein.The drawing figures are not necessarily to scale. Certain features ofthe invention may be shown exaggerated in scale or in somewhat schematicform and some details of conventional elements may not be shown in theinterest of clarity and conciseness. The present invention issusceptible to embodiments of different form. Specific embodiments aredescribed in detail and are shown in the drawings, with theunderstanding that the present disclosure is not intended to limit theinvention to the embodiments illustrated and described herein. It is tobe fully recognized that the different teachings of the embodimentsdiscussed herein may be employed separately or in any suitablecombination to produce desired results.

In the description below, various ranges and/or numerical limitationsmay be expressly stated below. It should be recognized that unlessstated otherwise, it is intended that endpoints are to beinterchangeable. Further, any ranges include iterative ranges of likemagnitude falling within the expressly stated ranges or limitations. Asused herein, the term “about” is meant to account for variations due toexperimental error. All numerical measurements are understood to bemodified by the word “about”, whether or not “about” is explicitlyrecited, unless specifically stated otherwise. Thus, for example, thestatement “a tensile strength of 70,000 psi” is understood to mean “atensile strength of about 70,000 psi.” In another aspect, “about” canmean within ±15% of the stated value, within ±10% of the stated value,within ±5% of the stated value, within ±2% of the stated value, orwithin ±1% of the stated value.

In the specification and appended claims, the terms “connect”,“connection”, “connected”, “coupled” and “coupling” are used to mean “indirect connection with” or “in connection with via another element.”

When describing the fabrication, formation or modification of an objectsuch as a spacer into a specific shape, the terms “shaped”, “reshaped”,“modified”, “modeled” or “remodeled” may be used interchangeably,regardless of whether the object was originally fabricated in that shapeor whether an object of a different shape was modified to take on thedescribed or disclosed shape.

The method and system as described are used to remove the heat ofreaction from a polyolefin production process. The coolant system asdescribed reduces surface friction and/or form friction between the heatexchange equipment and the coolant and surprisingly generate asignificant improvement in the pressure drop across the coolant system.The improvement in pressure drop across the system saves both energycosts, as well as capital costs, since smaller pumps may be effective tomove the coolant through the coolant system. By way of example,reduction of the pressure drop across the coolant system, i.e., thecombined pressure drop for all heat exchange equipment in the coolingsystem, from 15 psi to 11 psi can, depending upon the system, providesavings on the order of $100K to $1M per year.

Various aspects of this disclosure include one of more methods andprocesses for improving the efficiency of such as cooling system,including but not limited to, the following: [1] reducing the resistanceto coolant fluid flow by reducing the roughness of the internal surfaceof the jacket pipe, the external surface of the reactor pipe, thespacers within the annulus, or a combination thereof, for example, bypolishing or applying a friction-reducing coating; [2] shaping thespacers, particularly shaping the cross-section of the spacers, forimproved hydrodynamic performance and coolant fluid flow; [3] providingprotrusions or protuberances extending from the external surface of thereactor pipe and/or the internal surface of the jacket pipe into aportion of the annular space, including where the protrusions arematched with a corresponding protrusion on the opposite side of theannulus, particularly with a curved or partially curved leading edge,and a tapered or partially tapered trailing edge, to create a ventureeffect from the coolant flow; or [4] any combination thereof.

Therefore, in accordance with an aspect, this disclosure provides areactor system comprising:

-   -   a reactor pipe having an external surface characterized by an        unmodified surface roughness; and    -   a coolant system, the coolant system comprising        -   a jacket (that is, a jacket pipe) having an internal surface            characterized by an unmodified surface roughness, the jacket            spaced apart from and surrounding at least a portion of the            reactor pipe to form an annulus between the internal surface            of the jacket and the external surface of the reactor pipe,    -   a coolant which flows through the annulus to remove heat from        the reactor pipe, and    -   at least one spacer having a leading edge and a trailing edge        with respect to the coolant flow and characterized by an        unmodified surface roughness, the spacer coupling the jacket to        the reactor pipe;    -   wherein the external surface of the reactor pipe, the internal        surface of the jacket, the at least one spacer, or any        combination thereof are independently modified to reduce the        fluid resistance of the coolant flow through the annulus.

Therefore, according to an aspect, [1] the surface of the at least onespacer may be polished to smooth the surface, [2] the spacer may beshaped, that is modified from a square, rectangular, round cross orother conventional cross-sectional shapes, to provide a cross sectionthat is more efficient hydrodynamically, or [3] a combination thereof.The person of ordinary skill will understand that shaping or“remodeling” can be accomplished in any number of ways. For example,shaping can smooth or round the sharp edges of a square or rectangularcross section to the desired degree.

In an exemplary aspect, such a reactor system is seen in FIG. 1 , whichprovides an illustration of a loop reactor system for olefinpolymerization. A significant portion of commercial polyolefins arecurrently produced in slurry loop reactors, and FIG. 1 illustrates asimplified process flow diagram of an exemplary, non-limiting embodimentof a reactor system 10 suitable for use in association with the presentdisclosure. The reactor system 10 includes a loop reactor 14 composed ofsegments of loop reactor pipe connected by smooth bends or elbows. Thereactor 14 may be used to carry out polyolefin polymerization underslurry conditions in which insoluble particles of polyolefin are formedin a fluid medium and are suspended as slurry until removed.

In embodiments, the loop reactor system as described can have a volumeof at least about 35,000 gallons and/or a reactor production capacity ofat least about 125,000 lbs/hr, and a total coolant pressure drop throughthe reactor system of less than about 15 psi or less than about 11 psi,or a coolant pressure drop per leg of the loop reactor of less thanabout 1.4 psi. In an aspect the reactor system can have a productioncapacity of at least about 125,000 lbs/hr and a total coolant pressuredrop through the reactor system of less than about 15 psi.Alternatively, the reactor system can have a production capacity of atleast about 125,000 lbs/hr and a total coolant pressure drop through thereactor system of less than about 11 psi. The reactor system can alsohave a production capacity of at least about 125,000 lbs/hr and acoolant pressure drop per leg of the loop reactor of less than about 1.4psi. In embodiments, the loop reactor system can have a volume of atleast about 35,000 gallons and/or a reactor production capacity of atleast about 125,000 lbs/hr, and

In a further aspect, the reactor system can have a reactor capacity ofat least about 35,000 gallons and a total coolant pressure drop throughthe reactor system of less than about 15 psi. Alternatively, the reactorsystem can have a reactor capacity of at least about 35,000 gallons anda total coolant pressure drop through the reactor system of less thanabout 11 psi. The reactor system can also have a reactor capacity of atleast about 35,000 gallons and a coolant pressure drop per leg of theloop reactor of less than about 1.4 psi.

The liquid phase reactor, such as a loop slurry reactor system 10, maybe constructed of a material (e.g., high-strength aluminum) havinghigher strength and/or thermal conductivity than the steel materialstraditionally utilized in fabrication of the loop slurry reactor, Suchnewer high-strength materials may provide for improved thinner reactorwalls, increased heat-transfer through the walls, and a larger diameterof the loop reactor, permitting a higher polyolefin production rate. Yetanother example in the reactor system is the use of guide vanes in thereactor circulation pump system, providing for increased pumpingefficiency (reduced electrical consumption) and increased polyolefinproduction rate, A further example is a technique that specifies agreater increase in the temperature of the coolant flowing through thereactor jackets, for example, from the conventional 10° F. (−12° C.) tothe present range of from about 15° F. to about 45° F. (from about −9°C. to about 7° C.) and even higher. Such increased temperaturedifference between the coolant supply and return imparts substantiallythe same heat removal capability but at lower flow rates of coolant.

A motive device, such as a pump 18, circulates the fluid slurry in thereactor 14. An example of a pump 18 is an in-line axial flow pump withthe pump impeller disposed within the interior of the reactor 14 tocreate a turbulent mixing zone within the fluid medium. The impeller mayalso assist in propelling the fluid medium through the closed loop ofthe reactor, as depicted by arrows, at sufficient speed to keep solidparticulates, such as the catalyst or polyolefin product, suspendedwithin the fluid medium. The impeller may be driven by a motor 20 orother motive force. Solid polyolefin particulates may be removed fromthe reactor 14 via one or more settling legs (not shown) and/or othermeans, such as one or more continuous take-off (CTO) discharge means. Indownstream processing, the polyethylene discharged from the reactor maybe extracted from the slurry and purified.

Due to the exothermic nature of the polymerization reaction, coolingsystem 12 is provided to remove excess heat, i.e., the heat of reaction.Excess heat needs to be removed to achieve the desired degree ofpolymerization and the desired reaction speed while keeping thetemperature below that at which the polymer would go into solution, foulthe reactor, or reduce the heat transfer coefficient.

Coolant system 12 removes heat from the loop reactor 14 via reactorjackets 16A-16H. Coolant system 12 provides coolant supply 22 to reactorjackets 16A-16H and receives coolant return 24 from reactor jackets16A-16H. The coolant return 24 carries the heat removed from the reactor14. The coolant system 12 transfers this heat to a utility coolingmedium, such as to cooling tower water or sea water. The coolant system12 then delivers fresh coolant supply 22 to the reactor jackets. In anaspect, the features and methods of this disclosure are applicable tovertical loop reactors and horizontal loop reactors.

In an aspect, the coolant supply 22 temperatures may range from about85° F. to about 200° F. and the coolant return 24 temperatures may rangefrom about 115° F. to about 195° F., for example. Coolant may becirculated through the coolant system 12 and through the reactor jackets16A-16H, for example, by a centrifugal pump (not shown). Circulation ofcoolant requires a sufficiently sized pump to move the cooling fluidthrough the cooling jacket at the rate necessary to maintain sufficientheat exchange to control the polymerization process.

As provided in this disclosure, one method for improving the efficiencyof such as cooling system includes reducing the resistance to coolantfluid flow by reducing the roughness of the internal surface of thejacket pipe, the external surface of the reactor pipe, the spacerswithin the annulus, or a combination thereof, for example, by polishingor applying a friction-reducing coating. In this aspect, for example,the external surface of the reactor pipe, the internal surface of thejacket, the at least one spacer, or any combination thereof areindependently modified by polishing or by coating with afriction-reducing coating to reduce their unmodified surface roughnessto a respective modified surface roughness (R_(a)).

When any of the internal surfaces that contact the coolant duringcoolant flow are modified by polishing or by coating with afriction-reducing coating to reduce their unmodified surface roughness,the modified surface roughness (R_(a)) of the surface, such as theexternal surface of the reactor pipe, the internal surface of thejacket, the at least one spacer, or any combination thereof, can be0.00010 microns or less, 0.00008 microns or less, or 0.00006 microns orless, or any ranges therebetween. According to a further aspect, themodified surface roughness (R_(a)) of the external surface of thereactor pipe, the internal surface of the jacket, the at least onespacer, or any combination thereof is at least 50% less than, at least60% less than, or at least 70% less than the respective unmodifiedsurface roughness.

When polishing or by coating of the internal surfaces that contact thecoolant during coolant flow with a friction-reducing coating, themodified (polished and/or coated) surfaces can provide a total coolantpressure drop through the coolant system that is 20% lower, 18% lower,15% lower, or 12% lower than a corresponding total coolant pressure dropthrough an identical coolant system having the unmodified surfaceroughness. In a further aspect, the polished and/or coated spacers canhave a drag coefficient that is less than 1.5, less than 1.2, or lessthan 1.0.

FIG. 2 illustrates a cross-sectional view of jacketed section 16A asshown in FIG. 1 . The loop reactor system 200 comprises the loop reactorpipe 260 covered at least in part by coolant system jacket (or jacketpipe) 240 to form an annulus 220 between an internal surface 241 of thecoolant system jacket 240 and an external surface 261 of the loopreactor pipe 260 through which a coolant fluid flows (as shown by thedirectional arrows of FIG. 2 , generally illustrating fluid flow in theannulus 220) to remove heat of reaction from the loop reactor pipe 260.The heat of reaction is removed by contacting the coolant fluid with atleast a portion of the external surface 261 of the inner pipe 260. Thecoolant fluid generally also contacts at least a portion of the internalsurface 241 of the coolant system jacket 240 (also termed the reactorjacket) as well.

In a typical cooling jacket, spacers are provided approximately every 8to 12 feet to connect the cooling jacket to the reactor pipe. Typicalspacers comprise simple metal beams that are connected to both thereactor pipe 260 and the coolant system jacket 240. Conventional spacershave generally been rectangular or square in cross section, having foursides connected by 90-degree (90°) angles. These spacers, whilenecessary, retard fluid flow and result in a pressure drop across thecoolant system. Therefore, one or more aspects of the disclosure providefor shaping or remodeling the spacer and/or one or more surfaces in thecooling jacket to reduce drag resulting in less pressure loss, which canbe used in addition to or in place of the other disclosed methods forimproving cooling efficiency such as polishing the internal surfaces.

As described herein, and with continued reference to FIG. 2 , aplurality of spacers 210 and 215 are disposed within the annulus 220,between the reactor pipe 260 and the coolant system jacket 240. Thespecific embodiment illustrated in FIG. 2 shows a first section of lowerspacers 210 disposed within the lower section (i.e., the lower half ofthe height of the jacket, as designated by the height (H) of the coolantsystem jacket 240) of the annulus 220 and a second section of upperspacers 215 disposed within an upper section (i.e., the upper half ofthe height of the jacket, as designated by the height (H) of the coolantsystem jacket 240) of the annulus 220. According to one embodiment, in acommercial slurry loop reactor, the spacers used to secure the coolantsystem jacket 240 to the reactor pipe 260 can be periodic, for example,every 10 feet, along the entire length (or substantially the entirelength) of the vertical legs of slurry loop reactor 14. According to oneembodiment, the heat exchange coolant system jacket 240 can comprisegenerally any number of spacers, for example, at least 4 spacers, atleast 8 spacers, at least 12 spacers, at least 16 spacers, or, from 4 to24 spacers.

FIG. 3 illustrates a sectioned top view of spacers 215 from the loopreactor system 200 of FIG. 2 , taken along line 3-3. In the FIG. 3 view,only the upper spacers 215 disposed within an upper section (i.e., theupper half of the height of the jacket) are visible. Upper spacers 215are seen to couple the coolant system jacket 240 and the reactor pipe260 within the annulus 220. Because coolant flow is from behind the pagein FIG. 3 toward the viewer as seen in FIG. 2 , the top view of spacers215 is a view of the trailing edge of the spacers.

Therefore, one or more aspects of the disclosure include treating thereactor jacket and/or components thereof to reduce the surface roughnessthereof. Surface roughness, also referred to herein as “roughness”, is ameasure of the texture of a surface. It is quantified by the verticaldeviations of a real surface from its ideal form. Larger deviationsindicate more surface roughness, while smaller deviations indicate asmoother surface.

As used herein, the term “surface roughness” or “roughness” refers toaverage, one-dimensional surface roughness Ra. However, it is recognizedthat surface roughness can further be measured by alternativemeasurements, such as Rt (maximum height of the profile) and Rz (meanroughness depth (averaging of the distance between highest peaks andlowest valleys)), all of which may be reduced by embodiments describedherein. The profile roughness parameter Ra is the arithmetical meanroughness of a surface average of the roughness profile, specified bythe methods of standard test ISO/DIS 4287/1 and generally is reported inmicrons (μm). It can be represented by formula (I), below and determinedby the following equation:

R _(a=1/nΣ) _(i=1) _(n) _(|yi|)  (I)

The equation for roughness profile contains n ordered, equally spacedpoints along the trace, and yi is the vertical distance from the meanline to the ith data point. Height is assumed to be positive in the updirection, away from the bulk material.

In one or more embodiments, and in reference to FIG. 2 , the internalsurface of the reactor jacket 241, the external surface of the loopreactor pipe 261, the one or more spacers 210, 215 or a combinationthereof, can be treated (including treated by polishing) to reduce thesurface roughness (Ra) thereof. In one or more embodiments, the internalsurface of the reactor jacket 241 and one or more spacers are treated toreduce a surface roughness (Ra) thereof.

The treatment to reduce surface roughness may include any suitabletreatment known to one skilled in the art to reduce surface roughness ofa material. The treatment to reduce surface roughness can include, forexample, coating or polishing. Examples of polishing treatments mayinclude mechanical, electrochemical or chemical polishing. Likewise, thetreatment may include coating the surface with one or more materialsthat when dried form a smooth surface.

According to one embodiment, the treatment to reduce surface roughnessresults in a surface having a surface roughness that is at least 50%, orat least 60%, or at least 70%, or at least 75%, or at least 80%, or atleast 85%, or at least 90%, or at least 95%, or at least 98%, or atleast 99% lower than that of the untreated surface. Upon treatment, thesurface roughness of the treated material, such as the one or morespacers, the internal surface or combinations thereof, may be 1×10−4microns (□m) or less, or 0.5×10−3 microns or less, or 1×10−5 microns orless, or 0.5×10−4 microns or less, or 1×10−6 microns or less (asmeasured by ASTM D7127-13), for example.

The treatment to reduce surface roughness generally results in a totalcoolant pressure drop through the reactor system (i.e., the sum of allpressure drops through each reactor jacket/outer pipe within the reactorsystem) that is lower than an identical system absent the treatment toreduce surface roughness. For example, the total coolant pressure dropmay be at least 10%, or at least 15%, or at least 20%, or at least 25%,or at least 30%, or at least 35%, or from 15% to 90%, or from at least20% to at least 80% less than the total coolant pressure drop through anidentical reactor system absent the treatment to reduce surfaceroughness. In one or more specific embodiments, for reactors having avolume of at least about 35,000 gallons or a capacity of greater thanabout 125,000 lbs/hour, the total coolant pressure drop through thereactor system is less than 15 psi, or less than 13 psi, or less than 11psi, or less than 10 psi, or less than 9 psi, or less than 8 psi, orless than 7 psi, for example.

Further, treating the spacers to reduce surface roughness thereofgenerally results in a spacer pressure drop (i.e., the coolant pressuredrop as it passes over each spacer, such as from the leading edge to atrailing edge of the spacer) that is lower than that of an identicalspacer absent the treatment to reduce surface roughness. For example, inone or more embodiments, the total spacer pressure drop (i.e., the sumof all spacer pressure drops though each reactor jacket within thereactor system) is less than 20%, or less than 15%, or less than 12%, orless than 10% of the total coolant pressure drop through the reactorsystem. In one or more embodiments, the coolant pressure drop per leg ofthe loop reactor is less than about 2.0 psi (13.8 kPa), or less thanabout 1.75 psi (12.1 kPa), or less than about 1.6 psi (11.0 kPa), orless than about 1.5 psi (10.3 kPa), or less than about 1.4 psi (9.7kPa), or less than about 1.3 psi (8.9 kPa), for example.

Also provided in this disclosure is a method for improving theefficiency of such as cooling system by reducing the resistance tocoolant fluid flow, which can be accomplished by shaping the spacers,particularly shaping the cross-section of the spacers, for improvedhydrodynamic performance and coolant fluid flow. In this aspect, forexample, at least one spacer can be modified by shaping the spacer incross section to provide a curved or partially curved leading edge. Inanother aspect, at least one spacer is modified by shaping the spacer incross section to provide a curved or partially curved leading edge, anda tapered or partially tapered trailing edge.

When a spacer is shaped as described herein to improve theirhydrodynamic performance, the spacer can have a fineness ratio of atleast about 2, at least about 2.5, at least about 3.0 or at least about3.5. Moreover, when both the leading edge is curved or at leastpartially curved and the trailing edge is tapered or at least partiallytapered, the surface area of the curved or partially curved leading edgecan be greater than, equal to, or less than the surface area of thetapered or partially tapered trailing edge.

In a further aspect, the leading edge of a spacer can comprise a radiusof less than about 5.0 inches, less than about 4.5 inches, or less thanabout 4.0. In another aspect, the leading edge of a spacer can be shapedto reduce at least one radius by at least 30%, at least 37% or by atleast 45% from a 90° angle, that is, based upon a square or rectangularcross section with no radius applied to the 90° angle of the crosssection.

When spacers are shaped to be more hydrodynamic as described herein, andthe coolant system comprises a plurality of spacers configured asdisclosed, they can provide a venturi effect when the coolant flows overthe plurality of spacers. In this aspect, the pressure drop across anysingle spacer can be less than about 12%, less than about 10%, or lessthan about 8% of a total coolant pressure drop through the reactorsystem. In a further aspect, a spacer or a plurality of spacers can bemodified and/or shaping to have a drag coefficient of each modifiedand/or shaped spacer of less than 1.28, less than 1.22, or less than1.18.

This aspect of the disclosure of shaping the spacers is illustrated, forexample, in FIG. 4 and FIG. 5 . FIG. 4 illustrates a cross-sectionalarea of an exemplary spacer 400 according to one embodiment of thepresent disclosure, taken perpendicular to the length of the spacerbetween the loop reactor pipe 260 and the coolant system jacket 240, andviewed perpendicular to the direction of flow, and showing a curvedleading edge of the spacer. For example, FIG. 4 can represent across-sectional view of the spacers such as 210 and 215 of FIG. 2 andFIG. 3 . In the embodiment illustrated in FIG. 4 , the spacer 400 has aleading edge 402 and a trailing edge 404. Leading edge 402 in FIG. 4 isthe first portion of the spacer to contact the coolant as it circulatesthrough the annular space 220, and the trailing edge 404 in FIG. 4 , isthe final portion of the spacer to contact the coolant, as illustratedby coolant flow lines 405.

As illustrated in FIG. 4 , the leading edge 402 of the spacer has beenshaped to provide a more hydrodynamic shape for the coolant to passover. The leading edge 402 is shown shaped (or “modeled” or “remodeled”)to reduce at least one radius thereof. In the embodiment of FIG. 4 , theleading edge 402 results in a radius r₂ that is less than the originalradius thereof, r₁ of an unshaped 90° corner of a square or rectangularcross-section of a conventional spacer. In place of the unshaped 90°corner is a reshaped leading edge 402 which is reshaped to provide anarcuate profile in place of the 90° corners. Reshaping of the spacers inthis manner, typically by standard mechanical remodeling means, reducesthe form drag, sometimes referred to as the pressure drag, in fluid flowcaused by the spacers.

FIG. 5 illustrates a cross-sectional area of another exemplary spaceraccording to embodiments of the present disclosure, taken perpendicularto the length of the spacer between the loop reactor pipe 260 and thecoolant system jacket 240, and viewed perpendicular to the direction offlow and showing a curved leading edge and a tapered or partiallytapered trailing edge of the spacer. In FIG. 5 , spacer 500 has aleading edge 502 and a trailing edge 504. As seen in this embodiment,the spacer 500 has been shaped to provide arcuate profiles at both theleading edge 502 and the trailing edge 504. In this aspect, this shapemay be referred to as a curved or partially curved leading edge and atapered or partially tapered trailing edge. As illustrated in FIG. 5 ,coolant flow 505 passes more easily around the spacer 500 and forms asmaller wake than the wake formed when coolant flow passes around anon-shaped spacer or even the spacer 400 of FIG. 4 .

Referring still to FIG. 5 , radius r₁ represents the original radius ofan essentially square (or rectangular) spacer measured to a 90° corner.Radius r₂ represents the change in radius resulting from reshaping thecorners of the essentially square spacer to form arcuate areas on theleading edge of the spacer 502. Radius r₃ represents an alternativeradial length that results from reshaping the corner of the essentiallysquare (or rectangular) spacer on the trailing edge 504. As can be seenin the embodiment in FIG. 5 , the trailing edge 504 has a substantiallyshorter radius than the leading edge 502.

FIG. 6 and FIG. 7 show two possible embodiments of a hydrodynamic spacer210 or 215 having a leading edge 216 and a trailing edge 211. FIG. 6illustrates a portion of a spacer according to an embodiment of thepresent disclosure, showing a rounded, thicker leading edge 216 which iscontacted by the cooling fluid first and a rounded but narrower reartrailing edge of the spacer. FIG. 7 is similar to FIG. 6 , however thetrailing edge profile has been more significantly narrowed and taperedto improve its hydrodynamic profile. Therefore, FIG. 7 illustrates arear perspective view of a portion of a spacer according to anembodiment of this disclosure, showing the leading edge 216 which iscontacted by the cooling fluid first and much more substantially taperedand narrow rear trailing edge of the spacer as compared to the trailingedge of the spacer shown in FIG. 6 . This additional taper in the FIG. 7spacer can provide, for example, improved hydrodynamic efficiency.

When spacers are shaped to be more hydrodynamic as illustrated in FIG. 6and FIG. 7 , in some embodiments it is not necessary that the leadingedge of a spacer be thicker than the narrower trailing edge forenhancements in cooling efficiency to be realized. For example, in theembodiments illustrated in FIG. 6 and FIG. 7 , hydrodynamic efficienciesare also obtained when coolant flow is in the reverse direction fromthat described above. That is, coolant flow can occur from the a narrowleading edge of the spacer toward the thicker trailing edge of thespacer.

In a further aspect, for example, a plurality of spacers which areconfigured as disclosed may provide a venturi effect. Such an effect canoccur, for example, when there is a tapering from a thicker leading edgetoward a narrow trailing edge, or from a narrower leading edge toward athicker leading edge, when the plurality of spacers are configured andspaced apart to provide a venturi effect when the coolant flows over theplurality of spacers.

FIG. 8 illustrates another aspect of this disclosure, specifically, twopossible exemplary fluid flow patterns around differently shapedspacers, shown in cross section in FIG. 8 . Specifically, the FIG. 8cross-section is taken perpendicular to the length of the spacer betweenthe loop reactor pipe and the coolant system jacket pipe, and viewedperpendicular to the direction of flow. This figure demonstrates, forexample, that the drag coefficient of a streamlined body is lower whenthe boundary layer of the fluid around the body remains attached to thesurface of the body for as long as possible. A streamlined body with lowform drag will have a narrow wake, while a body with high form dragwould exhibit a broad wake. This is illustrated in FIG. 8 which shows afirst spacer 810 has a drag coefficient of 0.045, while the secondspacer 820 has a drag coefficient of 0.295. As can be seen from the flowlines, the wake at the end of the spacer 810 is smaller with little orno area of stagnation or pressure drop occurring at the trailing end ofthe spacer. Spacer 820 would have a larger wake and an area ofstagnation on the flat trailing end.

Current spacers produced from squared or rectangular beams have a dragcoefficient of approximately 2.0 at a theoretical laminar flow. Inembodiments as described, one or more spacers are shaped to provide adrag coefficient of less than 1.50, or less than 1.40, or less than1.30, or less than 1.28, or less than 1.25, or less than 1.20, or lessthan 1.10, for example less than 1.0, for example, less than 0.5, forexample, less than 0.1, when at theoretical laminar flow. Dragcoefficient is a dimensionless quantity that is used to quantify thedrag (or resistance) of an object in a fluid environment. One of skillin the art would understand that the measured parameters used tocalculate the dimensionless numbers referenced herein are converted toconsistent units which cancel out to provide the dimensionless number.For example, drag coefficient can be determined by the followingequation:

$C_{d} = {\frac{2F_{d}}{\rho V^{2}\frac{A}{2}}{or}\frac{F_{d}}{1/2\rho V^{2}A}}$

where F_(d) is the drag force (i.e., the force component in thedirection of the flow velocity) as measured in units known to oneskilled in the art, such as, for example, N; p is the density of thefluid as measured in units known to one skilled in the art, such as, forexample, kg/m³; V is the speed of the object relative to the fluid asmeasured in units known to one skilled in the art, such as, for example,m/s; and A is the reference area as measured in units known to oneskilled in the art, such as, for example, m².

According to another embodiment, the hydrodynamic profile of the spacerscan be defined by their fineness ratio. The term “fineness ratio”describes the overall shape of a streamlined body. As used herein, thefineness ratio is the ratio of the length of the spacer to its maximumwidth. The higher the fineness ratio, the more streamlined the body andthe lower the drag coefficient. According to one embodiment, the spacerhas a fineness ratio of at least about 2, for example, at least about 3,for example, at least about 4, for example, at least about 5, forexample, at least about 6, for example at least about 7. As the lengthof the spacer increases while the width of the spacer stays constant,the disruption of flow over the spacer will become smaller and the wakefollowing the spacer will be smaller. Fineness ratio is only one meansfor improving drag properties. Drag may also be affected by the angle ofincidence between the spacer and the flowing fluid.

FIG. 9 illustrates a partial, cutaway perspective view of an embodimentof the reactor system of the present disclosure, showing the reactorpipe 960, the jacket pipe 940, the annulus 920 therebetween, and aspacer 900 situated between the inside wall of the jacket pipe and theoutside wall of a reactor pipe. In this figure, fluid flow is frombottom to top of the annulus between the reactor pipe and the jacketpipe, and the therefore leading edge of the spacer is oriented downtoward the bottom of the figure. The downward-facing leading edge inFIG. 9 is thicker than the more narrow upward-facing trailing edge. FIG.10 illustrates a partial, cutaway perspective view of another embodimentof the reactor system of this disclosure, showing the reactor pipe 1060,the jacket pipe 1040, the annulus 1020 therebetween, and a spacer 1000situated between the inside wall of the jacket pipe and the outside wallof a reactor pipe, which can be understood by rotating the spacer fromFIG. 9 by 90° and altering the overall length distance from the leadingedge to the trailing edge, to provide the spacer of FIG. 10 .

According to one embodiment, the spacers may be disposed within theannulus in multiple horizontal planes. For example, the plurality ofspacers may be disposed in 2, 3, 4 or more horizontal planes within theannulus. According to another embodiment, spacers may be arranged out ofhorizontal alignment with one another and and/or not arranged inmultiple horizontal planes. For example, spacers may be arranged out ofhorizontal alignment by adding a pitch to the spacer. As used herein, apitch is the angle away from the horizontal assumed by the spacer.Referring to FIG. 2 , a spacer that starts at the reactor 260 andcrosses the annulus 220 perpendicular to the jacket has a pitch of zero.The same spacer crossing the annulus from the reactor 260 to the jacket240 at a 30-degree pitch has either a 30-degree angle or 150-degreeangle depending upon the assignment of the direction of the pitch.

This concept of pitch can be further demonstrated by reference to FIG. 5, which illustrates a cross-section of an exemplary spacer 500 takenperpendicular to the length of the spacer between the loop reactor pipeand the coolant system jacket, showing leading edge 502, trailing edge504, and coolant flow 505. When viewed in this cross-section, pitch canbe imparted to this spacer by rotating the spacer around the axiscoinciding with the length of the spacer which extends between the loopreactor pipe 260 and the coolant system jacket 240. In other words, whenviewed in the FIG. 5 cross-section, imparting pitch to spacer 500 isaccomplished by rotating the spacer about an axis extending out of andperpendicular the paper of FIG. 5 . In this aspect, for example, a pitchangle away from horizontal can be imparted to one or more spacers ofabout 5 degrees, about 10 degrees, about 15 degrees, about 20 degrees,about 25 degrees, about 30 degrees, about 35 degrees, or about 40degrees away from horizontal. Such pitch further deflects the fluid flowand when multiple spacers are similarly pitched, an improved effect oncoolant flow can be obtained. According to an aspect, a spacer also canhave a pitch of at least 5 degrees, for example, at least 10 degrees,for example at least 15 degrees, for example at least 20 degrees. Inanother aspect, a spacer has a pitch of from 1 degree to 30 degrees,from 3 degrees to 25 degrees, or from 5 degrees to 20 degrees.

The spacers may be formed of a variety of materials, such as ceramic orstainless steel. In one or more embodiments, the spacers can be formedof carbon or low-alloy steels, such as those illustrated in Table 1below.

TABLE 1 Thermal Minimum conductivity Tensile Steel (Btu/hr*F*ft)Strength (psi) A516 Gr70 27.8 70,000 (482.6 MPa) A537 Cl2 26.8 80,000A202 Gr B 23.9 85,000 A285 Gr C 30.1 55,000 A514 Gr B 27.4 110,000 A515Gr 70 27.2 70,000 A517 GR A 24.1 115,000 A517 Gr B 27.5 115,000 A533 TyA Cl 3 27.6 100,000 A542 Ty A Cl 2 21.5 115,000 A678 Gr C 25.7 95,000

Further, the spacers may be formed of a material having a minimumtensile strength of at least 50,000 psi (344.7 MPa). In one or moreembodiments, the spacers may be formed of a material having a minimumtensile strength of from 60,000 psi (413.7 MPa) to 90,000 psi (620.5MPa). Minimum tensile strengths are taken from “Lukens 1988-89 PlateSteel specification Guide,” Lukens Steel Co., Coatesville, Pa., 1988.

As described above, the cooling jackets 16A-16H or their components canbe fabricated or modified to improve the drag properties associated withform or pressure friction. According to an aspect, the cooling jacketscan be created or modified to improve drag due to surface friction, forexample, by polishing or coating with a friction-reducing coating.According to yet another aspect, the jackets and their components may beboth reshaped to be more hydrodynamic as well as being treated to reducesurface friction. Heat exchange jackets according to aspect have one ormore surfaces that are treated to improve the surface friction and oneor more reshaped spacers to thereby improve flow of the coolant andreducing or minimized pressure drop.

Referring again to FIG. 4 and FIG. 5 , aspects of shaping the spacer areillustrated. For example, in FIG. 4 and FIG. 5 , shaping or rounding ofthe leading edge is shown to enhance the hydrodynamic performance andefficiency of the spacer. For example, the edge may have a radius thatis smaller than unpolished spacers. For example, the treated edge, suchas the leading edge, the trailing edge, or a combination thereof, mayhave a radius of less than 5 inches (12.7 cm), or less than 4 inches(10.2 cm), or less than 3 inches (7.6 cm), or less than 2 inches (5.1cm), or less than 1 inch (2.5 cm), or less than 0.75 inches (1.9 cm), orless than 0.5 inches (1.3 cm), or less than 0.25 inches (0.6 cm). In oneor more embodiments, the treated edge is polished to reduce a radiusthereof by at least 10%, or at least 20%, or at least 25%, or at least30%, or at least 40%, or at least 50%, for example.

Also provided in this disclosure is another method for improving theefficiency of such as cooling system includes reducing the resistance tocoolant fluid flow by providing protrusions or protuberances extendingfrom the external surface of the reactor pipe and/or the internalsurface of the jacket pipe into a portion of the annular space. Thisaspect includes where the protrusions are matched with a correspondingprotrusion on the opposite side of the annulus, particularly with acurved or partially curved leading edge, and a tapered or partiallytapered trailing edge, to create a venture effect from the coolant flow.

Therefore, in accordance with a further aspect, this disclosure providesfor a reactor system, wherein at least one of the external surface ofthe reactor pipe and/or the internal surface of the jacket can beindependently further modified by the addition of one or moreprotrusions which extend from the internal surface of the jacket and/orthe external surface of the reactor pipe into a portion of the annularspace to create a venture effect from the coolant flow. In an aspect,the at least one protrusion can be shaped to provide a curved orpartially curved leading edge. In another aspect, the at least oneprotrusion can be shaped to provide a curved or partially curved leadingedge, and a tapered or partially tapered trailing edge.

FIG. 11 provides one illustration of this aspect of the disclosure,showing a cross-sectional area of a protrusion or protuberanceresembling a shaped bulge, the bottom of which is attached to the curvedsidewall of the internal surface of the jacket or the external surfaceof the loop reactor, and which extends into the annular space.Illustrated in FIG. 11 are protrusion 1100, leading edge 1102 ofprotrusion 1100, and trailing edge 1104 of protrusion 1100. In thisembodiment, protrusion 1100 is shaped to provide a curved leading edge1102 and a tapered or partially tapered trailing edge 1104. Fluid flowis from left-to-right in FIG. 11 , that is, from the leading edge 1102to the trailing edge 1104.

FIG. 8 is discussed above in terms of illustrating two possibleexemplary fluid flow patterns around differently shaped spacers,specifically, the cross-sectional areas of two exemplary spacers takenperpendicular to the length of the spacer. In an aspect, FIG. 8 can alsoillustrate a top view of two different shaped protrusions orprotuberances shown attached to the external wall of the reactor pipe orthe internal wall of the jacket pipe, demonstrating the improved fluidflow with the trailing edge is tapered and the leading edge is curved.The flow patterns in this instance are flow adjacent the wall to whichthe protrusion extends from and not over the top of the protrusion.

In a further aspect, the one or more protrusions can comprise or can beone or more continuous protrusions extending from the internal surfaceof the jacket and/or the external surface of the reactor pipe in acircular fashion. That is, a raised surface in the form or a ring orcircle can be added to the internal surface of the jacket and/or theexternal surface of the reactor pipe. The ring can have a curved orpartially curved leading edge, or in another aspect, the ring can havecurved or partially curved leading edge, and a tapered or partiallytapered trailing edge, with respect to coolant flow.

According to a further aspect, the reactor system can include acontinuous protrusion extending from the internal surface of the jacketand a corresponding continuous protrusion extending from the externalsurface of the reactor pipe which are spaced at substantially the sameheight along the jacket and reactor pipe, respectively, to provide aventuri effect when the coolant flows between the protrusions.

In still another aspect, this disclosure provides for one or morediscontinuous protrusions extending from external surface of the reactorpipe and/or the internal surface of the jacket pipe into a portion ofthe annular space in a non-circular and segmented fashion. In thisaspect, for example, the discontinuous protrusions can be situated andspaced at substantially the same height along the jacket and reactorpipe, respectively, to provide a venturi effect when the coolant flowsbetween the protrusions.

FIG. 12 illustrates this aspect of the disclosure, showing across-sectional area of one portion of the annular space between thecoolant system jacket 240 and the loop reactor pipe 260 and viewedperpendicular to the direction of coolant flow, according to anembodiment of the present disclosure This figure illustrates twoopposing protrusions or protuberances, 1200, the bottoms of which areattached to the curved sidewall of the internal surface of the jacket241 and the external surface of the reactor pipe 261, respectively, andboth of which extend into the annular space. Fluid flow is fromleft-to-right in FIG. 12 , that is, from the leading edge to thetrailing edge of the protrusions, and an exemplary venturi effect thatmay be generated between two the protrusions are shown. As the coolantpasses between the protrusions, the pressure increases and the speed ofthe fluid increases. As illustrated in FIG. 12 , the change in pressuresuch as the pressure drop can be measured by the coolant pressure beforethe narrow passage created by the protuberances, minus the pressure inthe narrow passage.

For example, when one discontinuous protrusion extends from the internalsurface of the jacket and a corresponding discontinuous protrusionextends from the external surface of the reactor pipe to comprise apaired set on opposite sides of the annulus. In this aspect, thediscontinuous protrusions resemble a series or set of isolatedprotuberances because they are non-continuous. In an aspect, forexample, the discontinuous protrusions comprise 2, 3, 4, 5, or 6 pairedset on opposite sides of the annulus. In these one or more embodiments,the leading edge can have has a surface area that is less than a surfacearea of the trailing edge.

In one or more embodiments, the coolant flows through the annulus at anaverage flow velocity of at least 20,000 ft³/hr (566 m³/hr), or at least25,000 ft³/hr (708 cm³/hr), or at least 27,000 ft³/hr (765 cm³/hr), orat least 30,000 ft³/hr (850 cm³/hr), or at least 35,000 ft³/hr (991cm³/hr), for example.

Sizing of appropriate pipes/jackets and the use of appropriate coolantsis well understood in the art. In one or more embodiments, the variouspipes defining the reactor passageway (i.e., loop reactor pipe) may havean outside diameter of from about 10 inches to about 30 inches and anominal wall thickness of from about 0.2 inches to about 1.0 inches, forexample.

In aspects, this disclosure provides a reactor system as describedherein, wherein the reactor system has a production capacity of at leastabout 125,000 lbs/hr and a total coolant pressure drop through thereactor system of less than about 15 psi or less than about 11 psi; andwherein at least a portion of the internal surface of the jacket istreated by polishing and has a surface roughness (R_(a)) at least 50%less than the corresponding untreated surface roughness. In this aspect,the surface roughness (R_(a)) can be 0.00010 microns or less.

Any suitable coolants may be used to remove or add heat to the reactorsystem. Typical coolants include steam condensate, water or combinationsthereof.

The improved coolant system can be used in conjunction with anypolyolefin process. A typical polyolefin process includes contacting anolefin monomer with a catalyst within a reactor system to form apolyolefin. The olefin monomers utilized in the processes describedherein may be chosen from C₂ to C₃₀ olefin monomers, or C₂ to C₁₂ olefinmonomers (e.g., ethylene, propylene, butene, pentene,4-methyl-1-pentene, hexene, octene and decene), for example. Themonomers may include olefinic unsaturated monomers, C₄ to C₁₈ diolefins,conjugated or nonconjugated dienes, polyenes, vinyl monomers and cyclicolefins, for example. Non-limiting examples of other monomers mayinclude norbornene, norbornadiene, isobutylene, isoprene,vinylbenzycyclobutane, styrene, alkyl substituted styrene, ethylidenenorbornene, dicyclopentadiene and cyclopentene, for example. The formedpolyolefin may include homopolymers, copolymers or terpolymers, forexample. In one or more embodiments, the olefin monomers are selectedfrom C₂-C₃ olefin monomers. In other embodiments, the olefin monomerincludes ethylene or propylene.

Catalysts may include any catalyst(s) or catalyst system(s) useful forpolymerizing olefin monomers. In this aspect, for example, the catalystmay be selected from chromium based catalyst systems, single sitetransition metal catalyst systems including metallocene catalystsystems, non-metallocene or post-metallocene catalyst systems, nickelcatalyst systems, Ziegler-Natta catalyst systems and combinationsthereof. As known in the art, the catalysts may be activated forsubsequent polymerization and may or may not be associated with asupport material.

The polymerization conditions including equipment, process conditions,reactants, additives and other materials will vary depending on thedesired composition and properties of the polyolefin being formed. Suchprocesses may include, for example, solution phase, gas phase, slurryphase, bulk phase, high pressure processes or combinations thereof.

A pre-polymer of controlled particle size distribution made using anycatalyst as described above may also be introduced to the polymerizationreactor. The prepolymerization may be carried out by any suitableprocess, for example, polymerization in a liquid hydrocarbon diluent orin the gas phase using a batch process, a semi-continuous process or acontinuous process. The conversion to prepolymer is generally carriedout by bringing the catalyst into contact with one or more alpha-olefinsin amounts such that the prepolymer contains between 0.002 and 10millimoles of transition metal per gram. The prepolymer particle sizemay be controlled by sieving, hydrocyclone or elutriation separation offines or large particles or other known techniques.

Embodiments may include a multiple reactor system wherein one of thereactors is a loop reactor and the second or any subsequent reactor ofthe multiple reactor system can be another loop reactor or can be anyreactor for the polymerization of olefins, for example a fluidized-bedreactor Similarly, the first reactor in a multiple reactor system inaccordance with this disclosure can be a fluidized-bed (gas-phase)reactor, and the second (or a subsequent) reactor can be one or moreloop reactors. The multiple reactor system can be used to make monomodalor multimodal polymers.

Upon removal from the reactor system, the polyolefin may be passed to apolymer recovery system for further processing, such as addition ofadditives and/or extrusion. Such recovery systems are known to oneskilled in the art and therefore are not described in detail herein.

Slurry phase processes (also referred to as particle formpolymerization) are common and generally include forming a suspension ofsolid, particulate polymer in a liquid polymerization medium, to whichmonomers and catalyst have been added. The suspension (which may includediluents) may be intermittently or continuously removed from thereaction zone, where the volatile components can be separated from thepolymer and recycled, optionally after a distillation, to the reactionzone. Liquefied diluent may optionally be employed in the polymerizationmedium and may be a diluent for the solid polymer particles that isseparate from and in addition to the unreacted monomers. Suitablediluents included those known in the art and include hydrocarbons whichare inert and liquid or are supercritical fluids under slurrypolymerization conditions. For example, suitable diluents may includeC₃-C₇ alkanes, such as isobutane, propane, n-pentane, i-pentane,neopentane and n-hexane. In one or more embodiments, the diluentincludes isobutane or isopentane. A bulk phase process is similar tothat of a slurry process with the exception that the liquid medium isalso the reactant (e.g., monomer) in a bulk phase process. However, aprocess may be a bulk process, a slurry process or a bulk slurryprocess.

The polymers (and blends thereof) formed via the processes describedherein may include, but are not limited to, linear low densitypolyethylene, elastomers, plastomers, high density polyethylenes, lowdensity polyethylenes, medium density polyethylenes, polypropylene andpolypropylene copolymers, for example. The polyolefins and blendsthereof are useful in applications known to one skilled in the art, suchas forming operations (e.g., film, sheet, pipe and fiber extrusion andco-extrusion as well as blow molding, injection molding and rotarymolding). Films include blown, oriented or cast films formed byextrusion or co-extrusion or by lamination useful as shrink film, clingfilm, stretch film, sealing films, oriented films, snack packaging,heavy duty bags, grocery sacks, baked and frozen food packaging, medicalpackaging, industrial liners, and membranes, for example, infood-contact and non-food contact application. Fibers includeslit-films, monofilaments, melt spinning, solution spinning and meltblown fiber operations for use in woven or non-woven form to make sacks,bags, rope, twine, carpet backing, carpet yarns, filters, diaperfabrics, medical garments and geotextiles, for example. Extrudedarticles include medical tubing, wire and cable coatings, sheets, suchas thermoformed sheets (including profiles and plastic corrugatedcardboard), geomembranes and pond liners, for example. Molded articlesinclude single and multi-layered constructions in the form of bottles,tanks, large hollow articles, rigid food containers and toys, forexample.

Additional details regarding loop reactor apparatus and polymerizationprocesses may be found, for example, in U.S. Pat. Nos. 4,674,290,5,183,866, 5,455,314, 5,565,175, 6,045,661, 6,051,631, 6,114,501,6,262,191, and 8,406,928, all of which are incorporated in theirentirety herein.

ADDITIONAL DISCLOSURE AND ASPECTS

The present disclosure is further illustrated by the followingembodiments, which are not to be construed in any way as imposinglimitations upon the scope thereof. On the contrary, it is to be clearlyunderstood that resort can be had to various other aspects, embodiments,modifications, and equivalents thereof which, after reading thedescription herein, can be suggestive to one of ordinary skill in theart without departing from the spirit of the present invention or thescope of the appended claims. Each and every claim is incorporated intothe specification as an embodiment of the present invention. Thus, theclaims are a further description and are an addition to the detaileddescription of the present invention.

Aspect 1. A reactor system comprising a reactor pipe having an externalsurface characterized by an unmodified surface roughness; and, a coolantsystem, the coolant system comprising a jacket having an internalsurface characterized by an unmodified surface roughness, the jacketspaced apart from and surrounding at least a portion of the reactor pipeto form an annulus between the internal surface of the jacket and theexternal surface of the reactor pipe, a coolant which flows through theannulus to remove heat from the reactor pipe, and at least one spacerhaving a leading edge and a trailing edge with respect to the coolantflow and characterized by an unmodified surface roughness, the spacercoupling the jacket to the reactor pipe; wherein the external surface ofthe reactor pipe, the internal surface of the jacket, the at least onespacer, or any combination thereof are independently modified to reducethe fluid resistance of the coolant flow through the annulus.

Aspect 2. The reactor system of Aspect 1, wherein the external surfaceof the reactor pipe, the internal surface of the jacket, the at leastone spacer, or any combination thereof are independently modified bypolishing or by coating with a friction-reducing coating to reduce theirunmodified surface roughness to a respective modified surface roughness(R_(a)).

Aspect 3. The reactor system of any one of Aspect 1, wherein theexternal surface of the reactor pipe, the internal surface of thejacket, the at least one spacer, or any combination thereof areindependently modified by polishing to reduce their unmodified surfaceroughness to a respective modified surface roughness (R_(a)).

Aspect 4. The reactor system of any one of the preceding Aspects,wherein modified surface roughness (R_(a)) of the external surface ofthe reactor pipe, the internal surface of the jacket, the at least onespacer, or any combination thereof is 0.00010 microns or less, 0.00008microns or less, or 0.00006 microns or less.

Aspect 5. The reactor system of any one of the preceding Aspects,wherein modified surface roughness (R_(a)) of the external surface ofthe reactor pipe, the internal surface of the jacket, the at least onespacer, or any combination thereof is at least 50% less than, at least60% less than, or at least 70% less than the respective unmodifiedsurface roughness.

Aspect 6. The reactor system of any one of the preceding Aspects,wherein the coolant system provides a total coolant pressure dropthrough the coolant system that is 20% lower, 18% lower, 15% lower, or12% lower than a corresponding total coolant pressure drop through anidentical coolant system having the unmodified surface roughness.

Aspect 7. The reactor system of any one of the preceding Aspects,wherein the at least one spacer is polished and has a drag coefficientof less than 1.5, less than 1.2, or less than 1.0.

Aspect 8. The reactor system of any one of the preceding Aspects,wherein the coolant system comprises a plurality of spacers, each havingan axis extending from the jacket to the reactor pipe, a portion ofwhich are disposed parallel to and approximately equidistant from eachother through the annulus.

Aspect 9. The reactor system of any one of the preceding Aspects,wherein the coolant system comprises a plurality of spacers, forexample, from 4 to 24 spacers.

Aspect 10. The reactor system of any one of the preceding Aspects,wherein the at least one spacer comprises a material selected fromceramic, stainless steel, or a combination thereof.

Aspect 11. The reactor system of any one of Aspects 1-10, wherein the atleast one spacer is modified by shaping the spacer in cross section toprovide a curved or partially curved leading edge.

Aspect 12. The reactor system of any one of Aspects 1-10, wherein the atleast one spacer is modified by shaping the spacer in cross section toprovide a curved or partially curved leading edge, and a tapered orpartially tapered trailing edge.

Aspect 13. The reactor system of any one of Aspects 11-12, wherein theat least one spacer has a fineness ratio of at least about 2, at leastabout 2.5, at least about 3.0 or at least about 3.5.

Aspect 14. The reactor system of any one of Aspects 11-13, wherein thesurface area of the curved or partially curved leading edge is greaterthan or equal to the surface area of the tapered or partially taperedtrailing edge.

Aspect 15. The reactor system of any one of Aspects 11-14, wherein thesurface area of the curved or partially curved leading edge is less thanthe surface area of the tapered or partially tapered trailing edge.

Aspect 16. The reactor system of any one of Aspects 11-15, wherein theat least one spacer has a pitch of from 1 degree to 30 degrees, from 3degrees to 25 degrees, or from 5 degrees to 20 degrees.

Aspect 17. The reactor system of any one of Aspects 11-16, wherein theleading edge of the at least one spacer comprises a radius of less thanabout 5.0 inches, less than about 4.5 inches, or less than about 4.0.

Aspect 18. The reactor system of any one of Aspects 11-17, wherein theleading edge is shaped to reduce at least one radius by at least 30%, atleast 37% or by at least 45% from a 90° angle.

Aspect 19. The reactor system of any one of Aspects 11-18, wherein thecoolant system comprises a plurality of spacers configured to provide aventuri effect when the coolant flows over the plurality of spacers.

Aspect 20. The reactor system of any one of Aspects 11-19, wherein thepressure drop across any single spacer is less than about 12%, less thanabout 10%, or less than about 8% of a total coolant pressure dropthrough the reactor system.

Aspect 21. The reactor system of any one of Aspects 11-20, wherein atleast a portion of the plurality of spacers is modified and/or shaped tohaving a drag coefficient of each modified and/or shaped spacer of lessthan 1.28, less than 1.22, or less than 1.18.

Aspect 22. The reactor system of any one of Aspects 1-21, wherein atleast one of the external surface of the reactor pipe and/or theinternal surface of the jacket are independently further modified by theaddition of one or more protrusions which extend from the internalsurface of the jacket and/or the external surface of the reactor pipeinto a portion of the annular space to create a venture effect from thecoolant flow.

Aspect 23. The reactor system of Aspect 22, wherein the at least oneprotrusion is shaped to provide a curved or partially curved leadingedge.

Aspect 24. The reactor system of Aspect 22, wherein the at least oneprotrusion is shaped to provide a curved or partially curved leadingedge, and a tapered or partially tapered trailing edge.

Aspect 25. The reactor system of any one of Aspects 22-24, wherein theone or more protrusions comprise one or more continuous protrusionsextending from the internal surface of the jacket and/or the externalsurface of the reactor pipe in a circular fashion.

Aspect 26. The reactor system of Aspect 25, comprising a continuousprotrusion extending from the internal surface of the jacket and acorresponding continuous protrusion extending from the external surfaceof the reactor pipe which are spaced at substantially the same heightalong the jacket and reactor pipe, respectively, to provide a venturieffect when the coolant flows between the protrusions.

Aspect 27. The reactor system of any one of Aspects 22-26, wherein theone or more protrusions comprise one or more discontinuous protrusionsextending from the internal surface of the jacket and/or the externalsurface of the reactor pipe in a non-circular and segmented fashion.

Aspect 28. The reactor system of Aspect 27, comprising a discontinuousprotrusion extending from the internal surface of the jacket and acorresponding discontinuous protrusion extending from the externalsurface of the reactor pipe which are spaced at substantially the sameheight along the jacket and reactor pipe, respectively, to provide aventuri effect when the coolant flows between the protrusions.

Aspect 29. The reactor system of Aspect 28, wherein the discontinuousprotrusions comprise a paired set on opposite sides of the annulus.

Aspect 30. The reactor system of Aspect 28, wherein the discontinuousprotrusions comprise 2, 3, 4, 5, or 6 paired set on opposite sides ofthe annulus.

Aspect 31. The reactor system of any one of preceding Aspects, whereinthe reactor system is a loop reactor comprising a series of reactor pipesections which form a series of legs and which form a loop.

Aspect 32. The reactor system of Aspect 31, wherein the reactor systemhas a production capacity of at least about 125,000 lbs/hr and/or areactor capacity of at least about 35,000 gallons, and a total coolantpressure drop through the coolant system of less than 15 psi, less than11 psi, or less than 9 psi.

Aspect 33. The reactor system of Aspect 31, wherein the reactor systemhas a production capacity of at least about 125,000 lbs/hr and/or areactor capacity of at least about 35,000 gallons, and a coolantpressure drop per leg of the loop reactor of less than 1.4 psi, lessthan 1.2 psi, or less than 1.0 psi.

Aspect 34. The reactor system of Aspect 31, wherein the reactor systemhas a reactor capacity of at least about 125,000 lbs/hr and a totalcoolant pressure drop through the reactor system is less than 15 psi;and wherein at least a portion of the internal surface of the jacket ispolished and the surface roughness (R_(a)) is reduced by at least 50%compared to an untreated surface roughness.

Aspect 35. A method of polymerizing olefins, the method comprisingcontacting at least one olefin monomer with catalyst within the reactorsystem of any one of the preceding Aspects under polymerizationconditions sufficient to form a polyolefin.

Aspect 36. A reactor system comprising:

-   -   a loop reactor comprising a reactor pipe having an external        surface and a volume of greater than about 35,000 gallons and        comprising a series of reactor pipe sections which form a series        of legs and which form a loop; and    -   a coolant system comprising        -   a jacket having an internal surface, spaced apart from and            surrounding at least a portion of the reactor pipe to form            an annulus between the internal surface of the jacket and            the external surface of the reactor pipe,        -   a coolant which flows through the annulus to remove heat            from the reactor pipe, and        -   a plurality of spacers, each having a leading edge and a            trailing edge with respect to the coolant flow and            characterized by an unmodified surface roughness, each            spacer coupling the jacket to the reactor pipe;    -   wherein the plurality of spacers are modified by shaping each        spacer in cross section to provide a curved or partially curved        leading edge, and a tapered or partially tapered trailing edge,        and the total coolant pressure drop through the coolant system        is less than 15 psi.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

We claim:
 1. A spacer for coupling a jacket to a reactor pipe of areactor system, the spacer comprising: a leading edge and a trailingedge with respect to coolant flow through the jacket and characterizedby an unmodified surface roughness; wherein the spacer is modified toreduce a fluid resistance of the coolant flow, and wherein the externalsurface of the spacer is modified by polishing or by coating with afriction-reducing coating to reduce the unmodified surface roughness toa respective modified surface roughness (R_(a)).
 2. The spacer of claim1, wherein the modified surface roughness (R_(a)) of the spacer is0.00010 microns or less.
 3. The spacer of claim 1, wherein the modifiedsurface roughness (R_(a)) of the spacer is at least 50% less than therespective unmodified surface roughness.
 4. The spacer of claim 1,wherein the spacer is polished and has a drag coefficient of less than1.5.
 5. The spacer of claim 1, wherein the spacer is modified by shapingthe spacer in cross section to provide a curved or partially curvedleading edge, a tapered or partially tapered trailing edge, or acombination thereof.
 6. The spacer of claim 5, wherein the spacer has afineness ratio of at least about
 2. 7. The spacer of claim 5, whereinthe spacer has a pitch of from 1 degree to 30 degrees.
 8. The spacer ofclaim 5, wherein: the leading edge of the spacer comprises a radius ofless than about 5.0 inches, and/or the leading edge is shaped to reduceat least one radius by at least 30% from a 90° angle.
 9. The spacer ofclaim 5, configured to provide a pressure drop across of less than about12% of a total coolant pressure drop through the reactor system.
 10. Thespacer of claim 5, wherein at least a portion of the spacer is modifiedand/or shaped to having a drag coefficient of less than 1.28.
 11. Ajacket for coupling to a reactor pipe of a reactor system and definingan annular space between the jacket and the reactor pipe, the jacketbeing characterized by an unmodified surface roughness, wherein aninternal surface of the jacket is modified to reduce a fluid resistanceof coolant flow through the annular space, and wherein the internalsurface of the jacket is modified by polishing or by coating with afriction-reducing coating to reduce the unmodified surface roughness toa respective modified surface roughness (R_(a)).
 12. The jacket of claim11, wherein the modified surface roughness (R_(a)) of the jacket is0.00010 microns or less.
 13. The jacket of claim 11, wherein at least aportion of the internal surface of the jacket is polished and thesurface roughness (R_(a)) is reduced by at least 50% compared to anuntreated surface roughness.
 14. A jacket for coupling to a reactor pipeof a reactor system and defining an annular space between the jacket andthe reactor pipe, the jacket being characterized by an unmodifiedsurface roughness, wherein an internal surface of the jacket is modifiedto reduce a fluid resistance of coolant flow through the annular space,and wherein the internal surface of the jacket is further modified bythe addition of one or more protrusions which extend from the internalsurface of the jacket into a portion of an annular space to create aventuri effect from the coolant flow.
 15. The jacket of claim 14,wherein the at least one protrusion is shaped to provide a curved orpartially curved leading edge, a tapered or partially tapered trailingedge, or both.
 16. The jacket of claim 14, wherein the one or moreprotrusions comprise one or more continuous protrusions extending fromthe internal surface of the jacket in a circular fashion.
 17. The jacketof claim 14, wherein the one or more protrusions comprise one or morediscontinuous protrusions extending from the internal surface of thejacket in a non-circular and segmented fashion.
 18. The jacket of claim17, comprising a discontinuous protrusion extending from the internalsurface of the jacket corresponding to a discontinuous protrusionextending from the external surface of the reactor pipe which are spacedat substantially the same height along the jacket and reactor pipe,respectively, to provide a venturi effect when the coolant flows betweenthe protrusions.
 19. A reactor pipe for coupling to a jacket of areactor system and defining an annular space between the jacket and thereactor pipe, the reactor pipe characterized by an unmodified surfaceroughness, wherein an external surface of the reactor pipe is modifiedto reduce a fluid resistance of coolant flow through the annular space,and wherein the external surface of the reactor pipe is modified bypolishing or by coating with a friction-reducing coating to reduce theunmodified surface roughness to a respective modified surface roughness(R_(a)).
 20. The reactor pipe of claim 19, wherein the modified surfaceroughness (R_(a)) of the reactor pipe is 0.00010 microns or less. 21.The reactor pipe of claim 19, wherein at least a portion of the externalsurface of the reactor pipe is polished and the surface roughness(R_(a)) is reduced by at least 50% compared to an untreated surfaceroughness.
 22. A reactor pipe for coupling to a jacket of a reactorsystem and defining an annular space between the jacket and the reactorpipe, the reactor pipe being characterized by an unmodified surfaceroughness; and wherein an external surface of the reactor pipe ismodified to reduce a fluid resistance of coolant flow through theannular space, and wherein the external surface of the reactor pipe isfurther modified by the addition of one or more protrusions which extendfrom the external surface of the reactor pipe into a portion of theannular space to create a venturi effect from the coolant flow.
 23. Thereactor pipe of claim 22, wherein the at least one protrusion is shapedto provide a curved or partially curved leading edge, a tapered orpartially tapered trailing edge, or both.
 24. The reactor pipe of claim22, wherein the one or more protrusions comprise one or more continuousprotrusions extending from the external surface of the reactor pipe in acircular fashion.
 25. The reactor pipe of claim 22, wherein the one ormore protrusions comprise one or more discontinuous protrusionsextending from the external surface of the reactor pipe in anon-circular and segmented fashion.
 26. The reactor pipe of claim 25,comprising a discontinuous protrusion extending from the externalsurface of the reactor corresponding to a discontinuous protrusionextending from the internal surface of the jacket which are spaced atsubstantially the same height along the jacket and reactor pipe,respectively, to provide a venturi effect when the coolant flows betweenthe protrusions.